Fifty years ago, dinosaurs lumbered across our screens like the slow-moving, shambling oafs they were thought to be. Now, they stride and sprint. They’re portrayed as active animals, often lithe and agile.

This popular makeover was inspired by a scientific one. Scientists used to believe that dinosaurs, like modern-day reptiles, were cold-blooded ectotherms. That doesn’t mean their blood was literally cold, but that they relied on the environment to heat their bodies. Many lines of evidence challenged this view, suggesting instead that some dinosaurs were warm-blooded endotherms like mammals and birds: they generated body heat by burning energy at a much greater rate than most reptiles.

This debate about dinosaur metabolism—were they ectotherms, endotherms or something in between?—is one of the longest-running in palaeontology. (“It feels like almost everyone working in dino palaeontology has weighed in on it at some point in their career,” says John Hutchinson.) Scientists have tried to address the issue by looking at the structure of their bones, the shape of their legs, the inferred anatomy of their lungs, the presence of insulating feathers, and the ratios of predators to prey. And some have tried to work out how quickly they grew.

Dinosaur bones are like tree trunks—they have rings, and each ring represents a year of growth. By studying specimens of different ages, scientists can work out how quickly these animals grew and how must energy they must have burned to do so. Bone rings give you growth rate, which gives you metabolic rate.

It’s an attractive approach because, unlike bone structure or the presence of feathers, growth can be quantified and clearly compared. “You can put a number on it,” says John Grady from the University of New Mexico. But these numbers come in drips, through piecemeal studies of a few species at most. Grady had grander aspirations.

His team has now amassed data on the growth rates of 381 animal species, both living and extinct, including 21 dinosaurs, 6 extinct crocodiles, and one prehistoric shark. He looked Tyrannosaurus and Apatosaurus, blue whales and deer mice, hammerhead sharks and Komodo dragons. He estimated each animal’s mass, growth rate, and metabolic rates, often refining (or completely reworking) mathematical models from earlier studies.

Lukewarm-blooded

His analysis revealed that dinosaurs sit somewhere between the endothermic and ecothermic extremes, epitomised by most mammals and reptiles. They couldn’t control their body temperature as precisely as a horse or human; equally, they weren’t as dependent on their environment as a snake or lizard. “The data pointed to dinosaurs not being quite like a reptile or a mammal, but to weird things like great white sharks, leatherback turtles, and tuna.”

Great whites and tuna are mostly cold-blooded but their hard-working muscles naturally heat their blood. In most fish, the warm blood would lose its heat as it travels to the gills for a dose of oxygen. But in these fish, the vessels are arranged so that the warm blood from the muscles travels past cold blood from the gills, and heats it up. They create body heat, and keep that heat in their bodies—a trick that can keep certain body parts up to 14 degrees Celsius hotter than the surrounding water,

The leatherback turtle uses similar heat exchangers, and it’s also very big. Big animals lose heat more slowly than small ones, so the leatherback has a sort of thermal inertia that keeps it warm.

Grady thinks that most dinosaurs used a similar strategy, which he calls mesothermy. They were lukewarm-blooded.

This isn’t just a wishy-washy middle-man term; it has a specific meaning. Endotherms use their metabolism to keep their body temperatures at a fixed point—excepting the occasional chill or fever, you’re almost always at 37 degrees Celsius. Ectotherms have more variable body temperatures and rely on the environment to heat themselves up. Some, like big crocodiles, are homeotherms—they rely on their size to keep a stable body temperature once they bask their way to warmth.

Mesotherms are different. Unlike a basking crocodile, they rely on their own metabolism to raise their body temperature. But unlike you, they don’t keep their temperatures at a fixed point. They turn the heating on, but they have no thermostats. Great whites and leatherbacks are good examples but mammals can be mesotherms too. The echidna—a spiny, egg-laying mammal from Australia—metabolises its way to an average temperature of 31 degrees Celsius, but that can vary by 10 degrees in either direction. It has a thermostat, but a very wobbly one.

Grady’s conclusion isn’t that new. Many, if not most, palaeontologists see the warm-blooded/cold-blooded debate as too simplistic. Instead, they believe there’s a continuum between these extremes, and dinosaurs fell somewhere in the middle. “There have been many studies arguing for intermediate metabolic rates in dinosaurs,” says Hutchinson, from the Royal Veterinary College, UK. “But this one stands out on its statistical treatment. It is very clear and testable, and it fits with other evidence.”

“To me, a lynchpin would be how this works for polar dinosaurs,” he adds. Grady’s team focused on species that lived in warm climates, and many dinosaurs lived in places with uncomfortable winters. Would a baby dinosaur living in a cold place still be mesothermic, or would it do something different? That’s something for the team to check next.

Variations

For now, Gregory Erickson from Florida State University, who has studied dinosaur growth, effusively praised the team’s attention to detail. “This is a remarkably integrative, landmark study [that] sets a new standard for growth research on extinct animals,” he says. “Now we can more rigorously compare how dinosaurs and the earliest birds grew relative to [living] animals and infer their metabolic status.”

Mieke Köhler from the Catalan Institute of Palaeontology is a bit more reserved. She notes that echidnas, tuna, and leatherbacks are all mesotherms, but control their body temperatures in very different ways. “They rely on completely different metabolic machinery,” she says. “They’re not a discreet group, but a collection of specialists that shifted their physiological state away from the extremes to converge somewhere in the middle.” By bundling the dinosaurs together under the same label, we risk whitewashing important differences in their lifestyles.

They were, after all, a very varied group. They dominated the planet for 185 million years, and there’s more time between Stegosaurus and Tyrannosaurus than between Tyrannosaurus and you. They diversified into forms both titanic and minute. Some had feathers and others didn’t. Some lived in the tropics and others lived in the freezing poles. If modern fish and mammals can vary in their physiology, they would have too. “There was probably variety, but I think many to most were mesotherms,” says Grady. “It makes sense of the conflicting back and forth evidence we’ve had. They’re not like modern birds or like reptiles.”

Even feathered dinosaurs like Archaeopteryx, which was either not quite a bird or just about a bird, came out as mesotherms. That surprised Grady. “This thing that was feathered like a bird wasn’t that much different to these non-feathered dinosaurs in how fast it grew,” he says. And it grew slowly! It took around 2 years for Archaeopteryx to reach adult size. A similarly sized hawk gets there in 6 weeks. “Its energy use was much lower than modern birds, but it was covered in feathers. Maybe it was an endotherm with a low metabolic rate, or something like the echidna. The jury’s still out.”

To Hutchinson, these results hint at a more interesting question than “Were dinosaurs warm-blooded or cold-blooded?” Instead, he would ask: “When did the ancestors of birds evolve a high metabolic rate?”

Grady also wonders if mesothermy could help to explain the long reign and frequent large size of dinosaurs. By raising their body temperatures, they could move their muscles faster and fire their nerves faster, becoming temporarily better at escaping or hunting. That’s why sharks and tuna do it. Swordfish and marlin can even warm up their brains and eyes to process information faster when they hunt.

But fully endothermic animals need to eat a lot to fuel their inner furnaces, which sets a limit to how big they can get. Grady wonders if mesothermy strikes a happy medium, allowing animals to stay competitive while also getting big.

PS: Dinosaur fans might be wondering about a recent controversy in which physicist Nathan Myhrvold challenged many published estimates of dinosaur growth rates, and argued that several papers contained serious flaws and discrepancies in their data. Grady’s paper was mid-way through the peer-review process when Myhrvold’s analysis landed, and he paid serious attention to it. But when he omitted data from the problematic papers (or even for problematic species), his results didn’t change. There are five pages of discussion on this in the supplemental materials for statisticians to pore over.

There are 13 Comments. Add Yours.

cjoyce
June 12, 2014

typo…grady is at university of new mexico. nice story tho

[Sorry. Fixed – Ed]

Zach Miller
June 12, 2014

I’m glad there’s more evidence for this mesothermy in dinosaurs. Scott Sampson’s “Dinosaur Odyssey” goes into this quite a bit, basically concluding that dinosaurs found a Goldilocks zone in which they were able to be active and fast-growing (compared to, say, crocodiles) but didn’t have the energy demands of giant mammals.

On the other hand, many studies have shown that sauropods grew very large very quickly, comparable to whales and/or birds. How does that group stack up in this new paper (I don’t have access to it)?

Zhou Chunfen
June 13, 2014

Excellent piece, as usual.
Paragraph 4 “…and how must energy they must have burned…” typo?

Kara Jones
June 13, 2014

There is no such thing as a mesothermic animal! That term applies to plants and I have no idea why the authors are trying to force it on to animals because it makes no sense. I’ve never seen it applied to animals before and for good reason: There are no “lukewarm” animals, just as there are no truly “cold-blooded” animals. In animal physiology, there are two components to thermoregulation: the source of heat (endothermy vs ectothermy) and the stability of body temperature (homeothermy vs poikilothermy). Endothermy simply means that the body is heated using metabolic heat. An animal can be any combination of the four terms. An animal can be an endotherm without being a homeotherm, and an animal can also be exhibit endothermy under certain conditions or in localized areas. For instance, tunas (one of the groups that the authors compare dinosaurs to) are all endotherms, but only bluefin tuna are homeotherms—the rest are poikilotherms (i.e. their body temperature varies and generally matches the surrounding environment). Echidnas are also endotherms, but they exhibit facultative poikilothermy during torpor and hibernation, which is a completely different strategy than any of the tunas. All the laminid sharks are also considered endotherms.

So long story short, all the animals the authors have defined as “mesotherms” are actually endotherms, so dinosaurs were endotherms! Large dinosaurs were probably homeotherms (since large size helps maintain a stable temperature, just as in the bluefin tuna) and small dinosaurs were probably facultative poikilotherms, maintaining high body temperatures so they could be active hunters during the day and conserving energy by allowing their bodies to match ambient temperatures during cold seasons and at night.

John Scanlon, FCD
June 13, 2014

“This isn’t just a wishy-washy middle-man term”
Well, no, I think it is. I totally agree with Kara regarding correct terminology, and though the misapplication of ‘mesothermy’ to vertebrates may be new, the use of weaselly phrases on this matter is pretty old. One example I made a note of (not unique): Angela Milner wrote in a 1996 book review (of ‘The Evolution and Extinction of Dinosaurs’ by David Fastovsky and David Weishampel) of a “growing consensus… that dinosaurs were physiologically unique – neither ectothermic like modern reptiles nor endothermic like birds and mammals”. This peculiar statement couldn’t mean “neither ectothermic nor endothermic” because that is a logical contradiction; almost nobody believed they were full-time ectotherms, but to honestly admit that all other possibilities imply endothermy would look like agreeing with Bob Bakker, or something crazy like that.
Very pleased that there’s a lot of new growth-rate data out, though!

Jura
June 13, 2014

Note: the poles of the Mesozoic were chilly (like modern-day Seattle), they were not freezing.

Nick Porter
June 14, 2014

This conclusion is based almost entirely on growth rates, yes?

That strikes me as flawed reasoning. Mammals and most birds raise their offspring almost or completely to adulthood, and the young are typically altricial, living comparatively sedentary lives in which their parents provide them with massively calorific diets. They grow very quickly because they can devote enormous amounts of energy towards it.

Most dinosaurs, on the other hand, seem to have been precocial. Some groups may have fended for themselves as soon as they hatched, others after a few weeks or months. Consequently, they couldn’t find as much nutrition as a mammal gets from milk, or a bird gets from its full-sized parent’s all-day foraging cycle.
Furthermore, they’d have to spend a lot of their energy on travelling, evading predators, finding shelter, finding their food, and digesting it. There’s not much left over for growing.

We tend to find that the dinosaurs with faster growth rates, such as ceratopsids, ornithopods and tyrannosaurids, are the ones which raised their young for a longer period of time.

There’s also a trend that animals’ growth rates begin to slow around the age when they start competing for mates or giving birth. This can be interpreted as a sign that activities which take up energy, leave less left over for the animal to grow.

My point being, growth rates are not on a fixed scale dictated solely by metabolism.

Hansen
November 27, 2014

“Mieke Köhler from the Catalan Institute of Palaeontology is a bit more reserved.”

In other words, she is more lukewarm.

Regarding the polar dinosaurs. There were ornithischian dinosaurs in the polar regions, with big eyes that makes some assume they were active during the winter. Because a large body conserves heat better than a small body, you would expect these animals to be big. But on the contrary, species like Leaellynasaura were small dinosaurs.

“They were, after all, a very varied group. They dominated the planet for 185 million years, and there’s more time between Stegosaurus and Tyrannosaurus than between Tyrannosaurus and you.”

That’s not totally correct. It would have meant that they dominated the planet right after the permian extinction. But they only originalted 230 million years ago, not 250 million years ago. But they did not become the dominant land animals before after the Triassic extinction. That should make the dinosaur era 135 million years long, not 185. But it’s still impressuve.

From an article that shows that dinosaurs would have more difficulties producing heat than mammals:
“The ability to produce brown fat evolved in a common ancestor of birds and mammals, but the ability to generate heat was lost in the group that gave rise to birds and lizards after it separated from the mammalian lineage (the researchers found the lizard genome similarly lacks a UCP1 gene). This strongly implies that dinosaurs, which diverged from birds even later than lizards, also lacked brown fat.”

Another article mentions a difference in the metablism between Enantiornithine birds, which were the dominating birds in the late Cretaceous, and modern birds (Neornithes):

“The difference in size of the ovarian follicles in each specimen records an evolutionary gradient from the large clutches of smaller eggs present in crocodilians and non-avian dinosaurs to the smaller clutches of large eggs present in living birds. The morphology of the ovarian follicles indicates that the yolks grew more slowly than those in living birds, consistent with the lower metabolic rate determined for Mesozoic birds through histological analyses that shows bone was deposited more slowly. Living birds grow very quickly and reach skeletal maturity before they become reproductively active. Histological analyses of these new specimens show that, like non-avian dinosaurs, basal birds became sexually mature before adulthood.”

Kara Jones
December 4, 2014

Hansen, the lack of brown fat doesn’t mean much. A wide variety of creatures are facultatively endothermic, either in portions of the body (flight muscles in insects), or most of the body (brooding in pythons, viviparous snakes). Facultative endothermy has evolved multiple times and evidence points to birds being able to use muscle-based non-shivering thermogenesis for heat production. Therefore, it is likely that at least some of the dinosaurs were able to produce heat endogenously, which could have increased their body temperatures to a level where they could function in sub-optimal external temperatures.

Hansen
December 4, 2014

But insects only produce heat when they use the wing muscles. To produce heat even when you are resting would require a different system if you don’t have brown fat. Being big is one advantage, and it could be one of the reasons why there were so few tiny dinosaurs. If they had a modified muscle system, then maybe further investigations of their skeletons can give a hint.

From another article about birds:
“This lineage was only spared from extinction, according to the TMH hypothesis, by selection for biochemical, physiological, and developmental novelties that facilitated enhanced thermogenesis and expansion of skeletal muscles.”

Kara Jones
December 4, 2014

Hi Hansen, I think you need to do some closer investigation into what non-shivering thermogenesis actually is. Some insects are able to decouple their flight muscles from movement, producing heat without moving their bodies (hence, why it’s called non-shivering, because they don’t move). Similarly, some birds are also able to produce heat through muscle-based non-shivering thermogenesis by activating a homologue of the UCP pathway found in mammals. The great thing about biology is that there is a thousand ways to skin a cat, not just one.

Hansen
December 4, 2014

I was already aware that insects often needs to warm up first by using their wing muscles to be able to fly, even without moving their bodies. And your comment about birds is already mentioned in the quote from the article I my previous post.

That’s gonna be my last post in this discussion. And for the future, please use a little less patronizing tone when sharing your knowledge with others.

Kara Jones
December 4, 2014

You said “But insects only produce heat when they use the wing muscles. To produce heat even when you are resting would require a different system if you don’t have brown fat.” This implies to me that you assumed that insects were moving when they were producing heat through their flight muscles, which they don’t. I’m sorry if I misunderstood what you were trying to say. You also said in your first post that birds lack the UCP1 gene, so I was simply trying to point out that there is a pathway being used that produces somewhat similar results through a different means (avUCP). I’m a biologist and studying all the unique adaptations that have evolved from animals responding to similar ecological pressures with innovative processes fascinates me. Most posters on here aren’t biologists and don’t have access to journal articles, so I was just trying to share information with you that I thought you hadn’t seen. I’m not sure why you find that patronizing.

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Ed Yong is an award-winning British science writer. His work has appeared in Nature, the BBC, New Scientist, Wired, the Guardian, the Times, and more. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible, regardless of their background.

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